With the rising cost of wood and the shortage of mature trees, there is a present need to find good quality substitutes for wood which will continue long into the future. Over the past several years a growing market has emerged for the use of polymer-wood composites to replace traditional solid wood products in applications such as decking, windows, fencing, automobile interiors and pallets. These composite materials typically consist of mixtures of thermoplastic materials with wood particles in the form of sawdust. The composite materials may be used in many of the same applications as an all-wood product but offer the advantages of providing enhanced resistance to: rot; attack by insects; and, deterioration due to the effects of moisture. These products can have the same workability as wood and are splinter-free.
Recently there has been an increased interest in composite materials as a viable outlet for recycled thermoplastic materials. This interest has been spurred by the prospect of environmental regulations mandating the recycling of these materials. Another benefit is that landfill space may be conserved if both spent sawdust and used plastic materials are reused rather than disposed of in a landfill.
U.S. Pat. No. 6,011,091 teaches a cellulosic composite containing from 60 to 70% cellulosic material, from 30 to 40% polyvinyl chloride material and up to 4% by weight of polar thermosetting material. U.S. Pat. No. 6,103,791 discloses a cellulosic composite comprising from 50 to 65% cellulosic material, from 25 to 30% polyvinyl chloride material and up to 4% by weight of polar thermosetting material.
U.S. Pat. No. 6,066,680 describes an extruded composite formed from foamable plastic containing a polymer resin and wood flour particles. The polymer resins described are polyvinyl chloride, chlorinated polyvinyl chloride and polystyrene. Published U.S. patent application Ser. No. 2003/0229160 describes composites comprising a polymer and a cellulosic fiber. Generally, conventional art describes the use of grafted-maleic anhydride polymers, or copolymers that incorporate a low level (less than 3 wt %) of functionality capable of forming covalent bonds with cellulosic material.
Maleic anhydride-grafted polymers (maleated polymers) and ethylene copolymers are known and conventional compatibilizers (also interchangeably referred to herein as “coupling agents”) for wood composites. Maleic anhydride-grafted polymers as used herein include maleated polyethylene, maleated polypropylene, maleated styrene-ethylene-butene-styrene triblock copolymer, and maleated polybutadiene. General discussions of anhydride coupling agents in wood composites can be found in: “Effectiveness of Functionalized Polyolefins as Compatibilizers for Polyethylene/Wood Flour Composites,” Y. Wang, F. C. Yeh, S. M. Lai, H. C. Chan, and H. F. Shen in Polym. Eng. and Sci. April 2003, vol. 43, n. 4, p. 933; and, “Surface of Cellulosic Materials Modified with Functionalized Polyethylene Coupling Agents,” Q. Li and L. M. Matuana in J. of Appl. Polym. Sci., (2003) vol. 88, p. 278.
The use of maleated polyolefins is not trouble-free, however. Grafting of maleic anhydride at a concentration of greater than two-weight % can be difficult and/or troublesome. To achieve higher levels of grafting, such as for example 3 wt %, it can be necessary to include a free-radical initiator in a higher concentration. Using the initiator at higher concentrations can cause undesirable side reactions, such as crosslinking and chain scission. Further, the cost of producing maleated polyolefins with high maleic graft levels can be substantial. Further, it can be problematical to produce maleic anhydride copolymers having 3 wt % maleic anhydride functionality by direct copolymerization. For example, U.S. Published patent application Ser. No. 2003/0021915 A1 discloses the use of Lotader 2210 and 3410, available from Atofina, which is an ethylene/maleic anhydride direct copolymer having less than 3 wt % maleic anhydride functionality.
It can be desirable for inclusion in a wood composite composition to have compatibilizers having included therein greater amounts of coupling functionality without the problems associated with higher levels of grafting in maleated polyolefins. A present need exists for a polymer-cellulosic fiber composite based on an easily processible polymer which has weatherability and dimensional stability which are at least at the same level as conventional products.
Non-grafted ethylene copolymers that include reactive functional groups such as maleic anhydride or a functional equivalent can be obtained by a high-pressure free radical process.